1. Field of the Invention:
This invention relates to production of cermet compositions wherein particles of a metal or metal alloy, are dispersed in a matrix of a ceramic material, and is particularly concerned with the production of cermet compositions of the above type, having high oxidation resistance, high abrasion and corrosion resistance, and good thermal shock resistance, and particularly designed for use as cermet seals for thermionic converters and diodes, and wherein the oxygen-impermeable barrier is provided around the metal or metal alloy particles, imparting oxidation resistance to such particles.
2. Description of the Prior Art:
It is known to strengthen high temperature resistant metals by adding ceramic or refractory metal oxides thereto. It is also known to incorporate high temperature resistant metals into ceramic materials or refractory metal oxides, in order to increase adhesion of the ceramic material or refractory metal oxide to a metal substrate, to provide high temperature resistant refractory coatings on such metal substrates.
In my U.S. application Ser. No. 348,422, filed Apr. 5, 1973, there is disclosed a high strength cermet composition comprising a sintered body consisting of high temperature resistant metal or metal alloy particles such as niobium or niobium alloys, and including iron and chromium, coated with and dispersed in a continuous matrix of a ceramic oxide such as alumina, yttria, zirconia or chromium oxide, and particularly designed for use as cermet seals for thermionic diodes. Although such cermet compositions have been found to be advantageous, cermets having even higher temperature oxidation resistance for application as seals for thermionic converters are required. Also, cermets having improved abrasion and corrosion resistance, coupled with high oxidation resistance are required for this application as well as for other applications including coal gasification equipment. superheaters, and the like.
Oxides are typically permeable to oxygen. Thus, when cermets comprised of metal particles and a ceramic oxide are fabricated, either this permeability of such oxides must be eliminated or the metal particles phase must be provided with oxidation resistance.
The article "High Reliability Protective Coatings For High Temperature Technology," by G. Parugini, et al., Paper 24 7th International Conference on Metal Spraying, September 10 - 14, 1973 London, England, discloses chromium-containing coatings on brazing materials such as nickel alloys, such coatings being resistant to chemical attack by oxygen and combusion gases. Such scale of Cr.sub.2 O.sub.3 is stated to adhere to the base metal, and a ceramic oxide such as zirconia can be applied over the chromium-containing coating.